Gas exchange - insects, single celled organisms, fish & leaf

Gas exchange in a single celled organism;

• large surface area to volume ratio
• oxygen absorbed by diffusion across body surface
• body surface only a cell surface membrane
• carbon dioxide from respiration diffuses out 
• cell wall is permeable - no barrier for gas exchange via diffusion.

Gas exchange in insects;

• insects need to conserve water to prevent dehydration from evaporation
• they have waterproof coverings on body surface
• they have a small surface area to volume ratio minimising area that water is lost.
• for gas exchange they have an internal network of tubes - tracheae
• these are supported by strengthened rings to prevent them collapsing
• these are then divided into smaller tubes called tracheoles which are extended throughout all body tissues - this brings oxygen to respiring tissues
• respiratory gases move in and out of tracheal system along a diffusion gradient and through ventilation (movement of muscles creating mass movements of air)
• gas enters and leaves tracheae through tiny pores - spiracles - on body surface
• spiracles are opened and closed by a valve. when open - water can evaporate so…